Preparation of nanoporous BiVO4/TiO2/Ti film through electrodeposition for photoelectrochemical water splitting

A nanoporous BiVO4/TiO2/Ti film was successfully fabricated by electrodepositing a nanoporous BiOI film on nanoporous TiO2 arrays followed by annealing at 450°C for 2 h. The electrodeposition of BiOI film was carried out at different times (10, 30, 100, 500 and 1000 s) in Bi(NO3)3 and KI solution. The morphological, crystallographic and photoelectrochemical properties of the prepared BiVO4/TiO2/Ti heterojunction film were examined by using different characterization techniques. UV–vis spectrum absorption studies confirmed an increase in absorption intensities with increasing electrodeposition time, and the band gap of BiVO4/TiO2/Ti film is lower than that of TiO2/Ti. The photocatalytic efficiency of BiVO4/TiO2/Ti heterojunction film was higher compared to that of the TiO2/Ti film owing to the longer transient decay time for BiVO4/TiO2/Ti film (3.2 s) than that of TiO2/Ti film (0.95 s) in our experiment. The BiVO4/TiO2/Ti heterojunction film prepared by electrodeposition for 1000 s followed by annealing showed a high photocurrent density of 0.3363 mA cm−2 at 0.6 V versus saturated calomel electrode. Furthermore, the lowest charge transfer resistance from electrochemical impedance spectroscopy was recorded for the BiVO4/TiO2/Ti film (1000 s) under irradiation.

A nanoporous BiVO 4 /TiO 2 /Ti film was successfully fabricated by electrodepositing a nanoporous BiOI film on nanoporous TiO 2 arrays followed by annealing at 4508C for 2 h. The electrodeposition of BiOI film was carried out at different times (10,30,100, 500 and 1000 s) in Bi(NO 3 ) 3 and KI solution. The morphological, crystallographic and photoelectrochemical properties of the prepared BiVO 4 /TiO 2 /Ti heterojunction film were examined by using different characterization techniques. UV-vis spectrum absorption studies confirmed an increase in absorption intensities with increasing electrodeposition time, and the band gap of BiVO 4 /TiO 2 /Ti film is lower than that of TiO 2 /Ti. The photocatalytic efficiency of BiVO 4 /TiO 2 /Ti heterojunction film was higher compared to that of the TiO 2 /Ti film owing to the longer transient decay time for BiVO 4 /TiO 2 /Ti film (3.2 s) than that of TiO 2 /Ti film (0.95 s) in our experiment. The BiVO 4 /TiO 2 /Ti heterojunction film prepared by electrodeposition for 1000 s followed by annealing showed a high photocurrent density of 0.3363 mA cm 22 at 0.6 V versus saturated calomel electrode. Furthermore, the lowest charge transfer resistance from electrochemical impedance spectroscopy was recorded for the BiVO 4 /TiO 2 /Ti film (1000 s) under irradiation.

Introduction
In the past few decades, photoelectrochemical (PEC) catalytic water splitting by using nanostructured semiconductors has been an effective way of producing hydrogen and oxygen [1,2]. 2. Experimental procedure 2.1. Preparation of nanoporous TiO 2 /Ti film Commercially pure titanium plates (0.5 mm thick, purity greater than 99.5%) were first degreased in acetone, mechanically polished, and finally, chemically polished at 258C in a solution consisting of H 2 O : HNO 3 : HF ¼ 6 : 3 : 1 (vol%) for 30 s. The pretreated titanium plates were anodically oxidized at 258C in 1 wt% hydrofluoric acid at 20 V for 15 min to produce nanoporous TiO 2 /Ti arrays. Then the nanoporous TiO 2 arrays were washed three times with de-ionized water and dried for the next step usage. The prepared nanoporous TiO 2 arrays were sintered in air at 4508C for 2 h to prepare the nanoporous TiO 2 /Ti film.

Characterization
The surface morphologies of heterojunction BiVO 4 /TiO 2 /Ti photoanodes were observed by Hitachi S-4700 field emission scanning electron microscopy (FESEM) after spraying the conducting layer with platinum. The bulk composition was investigated by energy-dispersive X-ray spectroscopy. The phases present in the coatings were characterized by a small angle diffractometric study carried out on a Riga KuD/max 2550PC X-ray automatic diffractometer. The optical performance of the asprepared materials was evaluated by using a UV-vis Lambda 750S in a wavelength ranging from 300 to 800 nm.
The PEC performance was evaluated in a three-electrode electrochemical cell with a quartz window to allow illumination. The working electrodes were the sintered nanoporous TiO 2 /Ti film and BiVO 4 / TiO 2 /Ti heterojunction film. SCE and Pt silk were used as the reference electrode and counter electrode, respectively. All the working electrodes were characterized in 0.2 M Na 2 SO 4 by CHI660E. Linear sweep voltammetry (LSV) was measured at a scanning rate of 0.01 V s 21 . Electrochemical impedance spectroscopy (EIS) was carried out under an open circuit voltage with frequencies ranging from 10 5 to 10 22 Hz with an AC voltage amplitude of 5 mV. The potentials in the I -V curves and in the PEC performance experiments were also controlled by CHI660E. A 150 W Xe lamp (Beijing Trust Tech Co. Ltd) was used to provide the visible light. EIS was used to explore the conductivity of the as-compared electrodes in dark and illumination environments in 0.2 M Na 2 SO 4 solution.

Results and discussion
The crystal structures of sintered nanoporous TiO 2 /Ti film and BiVO 4 /TiO 2 /Ti heterojunction films prepared with different electrodeposition times were characterized by X-ray diffraction (XRD) and are shown in figure 1. In figure 1a, the diffraction peaks at 2u of 29.68, 31.68, 45.38 and 51.38 can be indexed to BiOI (JCPDS no. 10 -0445). The as-anodized nanoporous TiO 2 /Ti films are of amorphous state due to the broad peak and only the peaks of the titanium substrate are present in the diffractogram [26]. After sintering at 4508C for 2 h, peaks at approximately 258 appear corresponding to anatase phase [26]. After modifying by BiVO 4  The morphology and nanostructures of BiVO 4 /TiO 2 /Ti heterojunction photoanodes were characterized using FESEM. Here, the morphology of TiO 2 /Ti film was not shown, because its morphology could be easily seen from the morphology of BiVO 4 /TiO 2 /Ti heterojunction photoanodes   where a is the absorption coefficient, v is the light frequency and E g is the band gap of a semiconductor. From the curve of (ahv) 2 versus hv shown in figure 3b, the energy of the band gap of TiO 2 /Ti film is 3.20 eV, which is the same as the result reported before [29].  Figure 4 shows the current-potential plots of nanoporous TiO 2 /Ti film and nanoporous BiVO 4 / TiO 2 /Ti heterojunction photoanodes under a 150 W Xe lamp illumination. The photocurrent densities of nanoporous TiO 2 /Ti film in the dark and under illumination were 0.02634 mA cm 22 and 0.0308 mA cm 22 at 0.6 V (versus SCE), respectively. The BiVO 4 /TiO 2 /Ti prepared with 10 s electrodeposition time under illumination exhibited a higher photocurrent density of 0.1583 mA cm 22 at 0.6 V (versus SCE) and showed a 403% higher photoactivity compared with bare TiO 2 /Ti film. Moreover, the photocurrent density of BiVO 4 /TiO 2 /Ti heterojunction photoanode increased with the increase in electrodeposition time in our experiment. When the electrodeposition time increased to 1000 s, the photocurrent density increased to 0.3363 mA cm 22 at 0.6 V (versus SCE). These results clearly indicate that the modification of nanoporous TiO 2 /Ti film with BiVO 4 effectively reduces the recombination of electrons and holes generated in the nanoporous BiVO 4 /TiO 2 /Ti film due to the formation of the heterojunction and excellent electron transport between TiO 2 film and the titanium substrate [30]. When BiVO 4 layers were coated on the TiO 2 surface, the light absorption range and intensity of BiVO 4 /TiO 2 /Ti films were improved and the electrons of BiVO 4 film could easily transfer to the nanoporous TiO 2 , resulting in a high photocurrent density.
The photocurrent response of photoanodes in the electrolyte directly correlates with the generation and transfer of the photo-excited charge carriers in the photocatalytic process [31]. The photocurrent responses of TiO 2 /Ti film and BiVO 4 /TiO 2 /Ti heterojunction films prepared with different electrodeposition times were investigated to enhance the charge separation in 0.2 M Na 2 SO 4 electrolyte at 0.6 V bias versus SCE. Both TiO 2 and BiVO 4 absorbed the photons and generated electron-hole pairs under the simulated sunlight illumination [32]. As shown in figure 5, it is clear that the photocurrent abruptly increased and decreased when the light source was switched on and off. The photoanodes of BiVO 4 /TiO 2 /Ti heterojunction films present obviously enhanced the photocurrent response compared with those of bare TiO 2 /Ti film. A photocurrent spike is clearly obtained in sudden illumination due to capacitive charging of the interface, and the spike decays because of recombination of the charge carriers associated with holes getting trapped at the surface [33]. When the simulated sunlight was turned on, the photocurrent density was a little higher than that 10 s later, which indicated the poor recombination abilities of photogenerated electrons with the holes in the TiO 2 modified with BiVO 4 electrodes [34]. It is obvious that the BiVO 4 /TiO 2 /Ti heterojunction films   with 1000 s electrodeposition time represent the highest photocurrent density compared to that of other photoanodes, which can be ascribed to the high nanoporous surface of BiVO 4 /TiO 2 /Ti heterojunction photoanodes and their excellent charge separation and transport properties. Thus, it can also be confirmed that the separation of electron-hole pairs was derived from the heterojunction [35]. The transient decay time can be analysed by a logarithmic plot of parameter D, using the following equation [33,36]: where I t is the current at time t, I s the stabilized current and I m is the current spike. The transient decay time can be defined as the time at which ln D ¼ 21 [37]. Figure  To evaluate the kinetics of the charge transfer process of the TiO 2 /Ti and BiVO 4 /TiO 2 /Ti photoelectrodes, EIS tests were carried out at 0.2 V versus SCE under a simulated solar light illumination. Figure 6a displays the Nyquist diagrams in the frequency range of 0.01 Hz to 100 kHz. In the plot, symbols indicate the experimental results and the inset picture is the magnified view of the Nyquist diagram of BiVO 4 /TiO 2 /Ti heterogeneous photoanodes. The arc in the Nyquist plot indicates the charge transfer kinetics on the working electrode. Obviously, the BiVO 4 /TiO 2 /Ti photoelectrodes present a lower charge transfer resistance, suggesting that the BiVO 4 /TiO 2 /Ti heterojunction facilitates charge transfer and separation. The simulated EIS results were obtained from the fitting procedures according to the ZSimpWin software, and the equivalent Randles circuit is shown in figure 6b. In the equivalent Randles circuit, Rs is the solution resistance, Qcpe is the constant phase element for the electrolyte/electrode interface and R is the charge transfer resistance across the interface of electrode/electrolyte. The arcs in the Nyquist plot are related to the charge transfer at the interface of the photoelectrode/electrolyte. The fitted values of R were 3183, 4373 and 322 000 V cm 22 for BiVO 4 /TiO 2 /Ti (1000 s), BiVO 4 /TiO 2 /Ti (100 s) and TiO 2 /Ti electrodes, respectively. The efficient charge transfer at the interface between photoelectrode and electrolyte hinders the charge recombination and induces the facile charge transport of electrons through the films. Thus, the bare TiO 2 /Ti film has a very low efficiency of charge transfer and shows the highest R value. The lowest R value for BiVO 4 /TiO 2 /Ti (1000 s) indicates that the charge transfer characteristics of BiVO 4 /TiO 2 /Ti heterojunction are good. Therefore, the modification of TiO 2 /Ti film with nanoporous BiVO 4 by forming the heterojunction could improve the charge transfer and photocatalytic ability of photoanodes.
The photogenerated electrons can move to the CB of TiO 2 from the CB of BiVO 4 easily owing to the type II heterojunction. The excited electrons in TiO 2 were facilely transported by the conductive Ti and directed to the Pt counter electrode via the external circuit (shown in figure 7). Therefore, the photogenerated electrons were scavenged by hydrogen ions on the Pt foil, while the photogenerated holes oxidized the water on the surface of the BiVO 4 /TiO 2 /Ti. The significantly enhanced PEC performance is attributed to the nanoporous structure, which improved the charge transport [38] and

Conclusion
A new type of nanoporous BiVO 4 /TiO 2 /Ti heterojunction photoanode was designed and fabricated by electrodepositing BiOI onto a TiO 2 /Ti nanoporous film followed by sintering at 4508C in vanadium (IV) oxy acetylacetonate solution for 2 h. A significant change was observed in the PEC properties of the BiVO 4 /TiO 2 /Ti heterojunction film by varying the electrodeposition time. The film electrodeposited for 1000 s showed a high photocurrent density of 0.3363 mA cm 22 at 0.6 V versus SCE. Furthermore, the lowest charge transfer resistance from electrochemical impedance spectroscopy was recorded for the BiVO 4 /TiO 2 /Ti heterojunction film electrodeposited for 1000 s under irradiation. Our results demonstrate that the nanoporous heterojunction BiVO 4 /TiO 2 /Ti photoanode is an effective design for improving the PEC performance owing to the excellent transport and separation efficiency. It will open a new opportunity for BiVO 4 /TiO 2 /Ti heterojunction photoelectrodes for water splitting by using solar energy.